1. Field of the Invention
The present invention relates to the industrial processes emitting gases or fumes which must be cleaned or purified before being rejected to the atmosphere. It relates more particularly to methods for wet cleaning or purifying gases or fumes of gaseous pollutants and particularly barely soluble gaseous pollutants.
2. State of the Art
The majority of industrial processes emit fumes containing different pollutants in particle and/or gaseous form. For collecting the dust, a large number of techniques have been proposed and carried out: cyclone, dry electrofilter, bag filter, venturi scrubber, wet electrofilter, etc . . . For collecting gaseous pollutants, dry and wet techniques have been proposed and carried out. In the dry techniques, a finely divided solid is injected into the fumes to be purified. The gaseous pollutants are then collected by the particles of said solid either by chemical reaction (for example HCl reacting on particles of lime) or by adsorption (for example mercury adsorbed on active charcoal). The finely divided solid is then collected in a deduster, for example electrofilter or bag filter.
All these techniques are limited by the difficulties inherent in the gas/solid transfers and lead to an excess consumption of reagents. This is why wet techniques are often preferred, in particular when the concentration to be attained in the purified fumes is low. In the wet techniques, an intimate contact is made between the fumes and the scrubbing liquid and the gaseous pollutants are transferred in said scrubbing liquid where they are either simply dissolved in the case of very soluble gases (for example HCl in water) or "neutralized" in the case of barely soluble gases by physico-chemical reaction with a reagent dispersed in the scrubbing liquid; the "neutralization" would make it possible to continue dissolution of the gaseous pollutant in question despite its weak solubility (for example SO.sub.2 "neutralized" in aqueous phase by lime or mercury "neutralized" by adsorption in aqueous phase on active charcoal). These wet techniques are generally more efficient than the dry techniques, but are also limited for example by the very weak solubilility of certain gaseous pollutants such as mercury metal or nitrogen monoxide. They are also limited by the conditions of collection imposed by the other, more soluble pollutants also present in the fumes to be purified. This is why it has been proposed to modify the chemical state of these gaseous pollutants, in gaseous phase, before collection of the pollutants. It has thus been proposed to inject sodium sulfide in the fumes which forms different sulfides with the pollutants present, particularly mercury sulfide with the mercury which is in particle form and may therefore be collected in a deduster placed downstream of the point of injection. The drawback of this method is due to the risk of release of hydrogen sulfide at the level of purification of the fumes or from the residues of purification of the fumes. Other methods have also been proposed to oxidize the nitrogen monoxide into nitrogen dioxide by injecting into the fumes different oxidizers such as chlorine dioxide, ozone, etc . . . After this step of oxidation in gaseous phase, the fumes are then purified by the wet method. These methods present the drawback of leading to a high consumption of reagents on the one hand due to the difficulty in mixing the oxidizer with a high fume flowrate, and on the other hand, by reason of the barely selective nature of the oxidation which may act for example on the sulfur dioxide very often present and which is more efficient and more economical to collect by other methods. In Patent FR-B-2 643 286, oxidation is effected more efficiently by injecting into the fumes, on the one hand, an easily soluble gaseous acid agent (for example HCl) and, on the other hand, downstream, by spraying, a scrubbing liquid containing an oxidation compound (for example NaClO.sub.2) which, when the HCl is absorbed in the droplets of liquid, reacts with the NaClO.sub.2 to generate upon contact with the droplets and fumes, an oxidizer, chlorine dioxide in the example in question. However, this method presents the drawback of necessitating a homogeneous mixture of the gaseous acid agent with a high fume flowrate upstream of the scrubber.
The present invention aims at overcoming these drawbacks while taking into account the requirements of an efficient multi-pollutant purification.